Naturally occurring extracellular matrices (ECMs) are used for tissue repair and regeneration. One such extracellular matrix is small intestine submucosa (SIS). SIS has been used to repair, support, and stabilize a wide variety of anatomical defects and traumatic injuries. Commercially-available SIS material is derived from porcine small intestinal submucosa that remodels to the qualities of its host when implanted in human soft tissues. Further, it is taught that the SIS material provides a natural matrix with a three-dimensional microstructure and biochemical composition that facilitates host cell proliferation and supports tissue remodeling. Indeed, SIS has been shown to contain biological molecules, such as growth factors and glycosaminoglycans that aid in the repair of soft tissue of the human body. The SIS material currently being used in the orthopaedic field is provided in a dried and layered configuration in the form of a patch to repair or regenerate soft tissue such as tendons, ligaments and rotator cuffs.
While small intestine submucosa is readily available, other sources of ECM are known to be effective for tissue remodeling. These sources include, but are not limited to, stomach, bladder, alimentary, respiratory, or genital submucosa, or liver basement membrane. See, e.g., U.S. Pat. Nos. 6,379,710, 6,171,344; 6,099,567; and 5,554,389, each of which is hereby incorporated by reference.
Further, while SIS is most often porcine derived, it is known that various submucosa materials may also be derived from non-porcine sources, including bovine and ovine sources. Additionally, the ECM material may also include partial layers of laminar muscularis mucosa, muscularis mucosa, lamina propria, stratum compactum and/or other such tissue materials depending upon factors such as the source from which the ECM material was derived and the delamination procedure.
As used herein, it is within the definition of a naturally occurring extracellular matrix to clean, delaminate, and/or comminute the extracellular matrix, or to cross-link the collagen or other components within the extracellular matrix. It is also within the definition of naturally occurring extracellular matrix to fully or partially remove one or more components or subcomponents of the naturally occurring matrix. However, it is not within the definition of a naturally occurring extracellular matrix to separate and purify the natural components or subcomponents and reform a matrix material from purified natural components or subcomponents. Thus, while reference is made to SIS, it is understood that other naturally occurring extracellular matrices (e.g. stomach, bladder, alimentary, respiratory, and genital submucosa, and liver basement membrane), whatever the source (e.g. bovine, porcine, ovine) are within the scope of this disclosure. Thus, in this application, the terms “naturally occurring extracellular matrix” or “naturally occurring ECM” are intended to refer to extracellular matrix material that has been cleaned, processed, sterilized, and optionally crosslinked. The following patents, hereby incorporated by reference, disclose the use of ECMs for the regeneration and repair of various tissues: U.S. Pat. Nos. 6,379,710; 6,187,039; 6,176,880; 6,126,686; 6,099,567; 6,096,347; 5,997,575; 5,993,844; 5,968,096; 5,955,110; 5,922,028; 5,885,619; 5,788,625; 5,762,966; 5,755,791; 5,753,267; 5,733,337; 5,711,969; 5,645,860; 5,641,518; 5,554,389; 5,516,533; 5,460,962; 5,445,833; 5,372,821; 5,352,463; 5,281,422; and 5,275,826.
The manipulation of scaffold pore size, porosity, and interconnectivity is an important science contributing to the field of tissue engineering (Ma and Zhang, 2001, J Biomed Mater Res, 56(4):469–477; Ma and Choi, 2001 Tissue Eng, 7(1):23–33) because it is believed that the consideration of scaffold pore size and density/porosity influences the behavior of cells and the quality of tissue regenerated. In fact, several researchers have shown that different pore sizes influence the behavior of cells in porous three-dimensional matrices. For example, it has been demonstrated in the art that for adequate bone regeneration to occur scaffold pore size needs to be at least 100 microns (Klawitter et al., 1976, J Biomed Mater Res, 10(2):311–323). For pore sizes and interconnectivity less than that, poor quality bone is regenerated and if pore size is between 10–40 microns bone cells are able to form only soft fibro-vascular tissue (White and Shors, 1991, Dent Clin North Am, 30:49–67). The consensus of research for bone regeneration indicates that the requisite pore size for bone regeneration is 100–600 microns (Shors, 1999, Orthop Clin North Am, 30(4):599–613; Wang, 1990, Nippon Seikeigeka Gakki Zasshi, 64(9):847–859). It is generally known in the art that optimal bone regeneration occurs for pore sizes between 300–600 microns.
Similarly, for the regeneration of soft orthopaedic tissues, such as ligament, tendon, cartilage, and fibro-cartilage, scaffold pore size is believed to have a substantial effect. For example, basic research has shown that cartilage cells (chondrocytes) exhibit appropriate protein expression (type II collagen) in scaffolds with pore sizes of the order of 20 microns and tend to dedifferentiate to produce type I collagen in scaffolds with nominal porosity of about 80 microns (Nehrer et al., 1997, Biomaterials, 18(11):769–776). More recently, it has been shown that cells that form ligaments, tendons, and blood vessels (fibroblasts and endothelial cells) exhibit significantly different activity when cultured on scaffolds with differing pore sizes ranging from 5 to 90 microns (Salem et al., 2002, J Biomed Mater Res, 61(2):212–217).